1. Field of the Invention
The invention relates to a method and apparatus for manufacturing an electrode for a polymer electrolyte fuel cell.
2. Description of Related Art
A polymer electrolyte fuel cell is formed as a stack including a lamination of cells (cell lamination), terminals (electrode plates), insulators, end plates, a fastening member (e.g., a tension plate). More specifically, the cell lamination is formed by laminating a plurality of membrane-electrode assemblies (MEAs) and separators. A terminal (electrode plate), an insulator and an end plate are provided at both ends of the cell lamination in the laminating direction. A fastening member tightens the cell lamination in the laminating direction, and is provided outside the resultant cell lamination so as to extend in the laminating direction. Each MEA is formed from an electrolyte membrane, an anode provided on one surface of the electrolyte membrane, and a cathode provided on the other surface thereof. The electrolyte membrane is formed from an ion-exchange membrane. The anode and cathode have a catalyst layer. Diffusion layers are respectively provided between the catalyst layers and the separator. Each separator has fluid passages for supplying a fuel gas (hydrogen) and an oxidizing gas (oxygen; usually air) to a corresponding anode and cathode, respectively.
In the polymer electrolyte fuel cell, the following reactions occur at the anode and cathode: at the anode, hydrogen ions and electrons are produced from hydrogen. The hydrogen ions thus produced travel toward the cathode through the electrolyte membrane. On the other hand, at the cathode, water is produced from oxygen, hydrogen ions and electrons (the electrons are produced at the anode of an adjacent MEA and then reach the cathode through the separator, or produced at the anode of the cell located at one end of the cell lamination and reach the cathode through external circuit).
Anode: H2→2H++2e−
Cathode: 2H++2e−+(1/2)O2→H2O
A commonly used electrolyte membrane has a thickness of about 10 to 100 μm. Each catalyst layer has a thickness of about 1 to 10 μm. The catalyst layer is applied to both surfaces of the electrolyte membrane or one surface of the diffusion layer (which is formed from carbon paper or carbon cloth).
An electrode (anode, cathode) material may be applied to the electrolyte membrane by any of the following methods:
A. a wet application method for directly applying the electrode material to the electrolyte membrane by printing, roller coating, spraying or the like;
B. a method for applying a catalyst layer to a polytetrafluoroethylene sheet or the like, attaching this catalyst layer to the electrolyte membrane by heat transfer (hot pressing), and then removing the polytetrafluoroethylene sheet; and
C. a special application method disclosed in Japanese Patent Laid-Open Publication No. 3-295168, that is, a method for electrostatically attaching an electrode material of a fuel cell to the whole surface of the electrolyte membrane.
However, the above direct application methods A, B have the following problems:
A. The direct application method is based on a wet application method in which an electrode material, that is, carbon powder having a noble metal attached thereto, is dissolved or suspended in a solvent such as isopropyl alcohol, ethanol or xylene for application. Since the solvent may alter the electrolyte membrane or may cause swelling and shrinking thereof, cracks are likely to be generated in the applied electrode layer.
Moreover, depending on the kneading conditions of the electrode material and the solvent, the electrode material and the solvent may not be uniformly kneaded, thereby possibly producing lumps of carbon powder and thus making uniform application difficult.
Moreover, it is impossible to produce an electrode having an arbitrary shape or an electrode with its structure (such as concentration) varied depending on the regions within a prescribed shape.
B. In the wet application method, the catalyst layer applied to a sheet is transferred to the electrolyte membrane. This method requires the transfer step. This increases the number of steps and also complicates the manufacturing process, causing increase in costs. Moreover, it is impossible to produce an electrode having an arbitrary shape or an electrode with its structure (such as concentration) varied depending on the regions within a prescribed shape.
The method of Japanese Patent Laid-Open Publication No. 3-295168 is based on a dry application method. Therefore, unlike a wet application method, cracks or the like are not generated in the electrode due to the reaction between a solvent and the electrolyte membrane and swelling and shrinking of the electrolyte membrane caused by the solvent. However, the above method cannot produce an electrode having an arbitrary shape and an electrode with its concentration varied depending on the regions within a prescribed shape and/or with its composition varied in the thickness direction of the electrode.